Electromagnetic horn antenna



April 1, 1952 w. c.' WILKINSON, JR 2,591,486

ELECTROMAGNETIC HORN ANTENNA Filed Dec. 31, 1949 2 SHEETS-SHEET 1.

'II'I'IIIIIIIIIIA INVENTOR William 61 Y/g illu' $011, J1: m

ATTC R N EY April 1, 1952 w. c. WILKINSON, JR

ELECTROMAGNETIC HORN ANTENNA 2 SHEETSSHEET 2 Filed Dec. 51, 1949 IN\ENTOR 1111.912] amlkm W ATTORNEY fatented Apr. 1 1952 ELECTROMAGNETIC HORN ANTENNA William C. Wilkinson, Jr., Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1949, Serial No. 136,179

15 Claims.

This invention relates to electromagnetic energy radiators or receptors, and more particularly to horn radiators or receptors having a flared aperture or opening.

Radiators or receptors having horn apertures are well known in the art, and such devices have found application in radio echo and detection apparatus. One of the difliculties encountered in the operation of radiators or re ceptors having a flared aperture is that when the aperture is appropriately flared to match the device to outer space, or to secure a desired radiation beam width, or both; the radiation pattern thereof usually has side lobes. Such side lobes in the radiation or response pattern are generally undesirable and are particularly undesirable in radio echo detection systems because they tend to cause the apparatus to give a false indication under many circumstances as will be understood by those familiar with such apparatus. When means are employed to suppress or reduce these side lobes, the resultant arrangement may be frequency sensitive; at some frequencies the side lobes are reduced and at others within the operating frequency range they may even be increased.

It is an object of the present invention to provide an improved radiator or receptor of the flared aperture or horn type.

It is another object of the invention to improve the radiation pattern of radiators or receptors of the type mentioned.

It is a further object of the invention to reduce side lobes in the radiation pattern of such radiators or receptors.

It is still another object of the invention to suppress the side lobes of such radiators or receptors over a wide band of operating frequenc1es.

Another object of the invention is to make a compact horn radiator or receptor with negligible side lobing.

A still further object is to attain the foregoing objects with a relatively compact arrangement.

These and other objects, advantages, and novel features of the invention will be more apparent from the following description when taken in connection with the accompanying drawing in which like reference numerals refer to like parts and in which:

Fig. 1 is a perspective view of a so-called "pillbox antenna having a flared or horn aperture and constructed according to the prior art;

Fig. 2 is a cross-sectional view along the lines 2-2 of Fig. 1;

Figs. 3, 4, and 5 are cross-sectional views of modifications of the arrangement of Figs. 1 and 2 Fig. 6 is a cross-sectional view of an embodiment of the invention in which the structure of Figs. 1 and 2 is modified in accordance with the teachings of the invention;

plates.

Fig. 7 is a cross-sectional view of another modiflcation of the structure of Figs. 1 and 2 according to the invention and showing certain constructional details; and

Fig. 8 is a graph of radiation patterns illus-- trating the improvement secured by the practiceof the invention.

A typical embodiment of the invention com-- prises a radiator of the type described having a horn aperture, with a reflector closely spaced. near the wall portion of the horn mouth. If the reflector is metallic, it is dielectrically spaced and insulated from the walls of the mouth and in any event extends only partially beyond the wall portions of the mouth or aperture in the principal direction of energy radiation, to reduce or eliminate'the side lobes in the radiation pattern. In a preferred form of the invention, a pillbox horn having its sides flared in the plane of the electric vector, is provided with a dielectric reflector close to and extending only partially beyond the sides of the flared aperture :in the direction of radiation. The reflectors preferably are substantially planar and preferably have energy absorptive material on the side away from that central plane of the radiation which is normal to the electric vector. The side-lobing is thereby practically eliminated and these arrangements in accordance with the invention are not highly frequency sensitive with respect to side lobe characteristics.

Referring now more particularly to Figs. 1

and 2, which are respectively perspective and cross-sectional views of a prior art structure, a waveguide l0 feeds a so-called pillbox radiator l2 having substantially parallel plates [4 and I6 separated preferably by less than a half wavelength at the operating frequency. The pillbox l 2 has a parabolic cylindrical reflecting back wall 18 normal to the planes of plates [4 and I6. Waveguide l0 feeds the energy through an aperture 20 in wall M in a direction normal to the The feed energy from waveguide I0 is reflected by a metallic reflector 22 placed substantially at the focal point of the parabolic cylin drical reflector I8, and disposed at an angle of 45 with respect to the plates M, I 6 to provide efficient reflection of the energy introduced from the waveguide Ill. The energy in waveguide 10 is polarized in the direction indicated by the arrow 24, and after reflection, the energy is polarized in a direction normal to the parallel plates l4, Hi.

The energy reflected from the reflector 22 strikes the parabolic cylindrical reflecting surface IB and is again reflected in a substantially plane wave front toward the flared wall. portions or horn aperture 26 which is formed by two substantially planar metallic plates 29 and 30 joined respectively to the plates l4 and I6 and 3 disposed at an acute angle to the central plane 32 of the aperture-26.

In operation, this prior art structure has a radiation pattern in a central plane normal to the central plane 32 substantially as shown by the somewhat idealized dotted line curve 34 of Fig. 8 which is a plot of field strength on a decibel scale at a point considerably distant (many wavelengths) from the device with respect to the angle from the central plane 32 as measured from the horn mouth to the point. Graph 34 includes the rather prominent side lobes 33. The principal direction of radiation is the intersection of these two central planes, in one of which lies the axial plane of the parabolic cylinder Ill. The radiation may be considered as focused or restricted by the flared sides in the E plane to a principal lobe having a central plane of symmetry (the same plane as central plane 32) normal to the electric vector of the radiated energy.

The side lobes 33 may be reduced by the arrangement of Fig. 3 which is a cross-sectional View of the mouth portion of the pillbox l2 further provided with two planar metallic plates 36 and 38 which are joined respectively to the flared sides or walls 28, 38 at their junction with the plates l4, H6. The plates 36, 38 extend in the principal direction of radiation beyond the ter mination of the plates 28, 30. By appropriate adjustments of the angles between the plates 28 and 36 and the pltaes 30 and 38, and the extension of plates 36, 38 beyond the termination of plates 28, 30 the side lobes 33 of Fig. 8 are reduced. Referring now more particularly to Fig. i, there is shown an alternative to the arrangement of Fig. 3 in which metallic planar plates 40 and 42 are disposed substantially parallel to the plates 28 and 36 respectively and extend only partially beyond the terminations of the latter in the principal direction of energy radiation. They maybe joined to plates 28 and 30 and supported by me tallic members-44 and 46, respectively, which extend the length of the plates 28 and 3B in a direction normal to the cross-section of Fig. 4.

A further arrangement alternative to that of Figs. 3 and 4 is illustrated in Fig. in which planar plates 43 and 59 are disposed at an acute angle to the plates 28 and 3%! but are joined to these plates at a point more in advance of the junction of the latter with the plates l4 and it.

As in the embodiment of Fig. 4, they extend only partially beyond the termination of plates 28 and 30 in the principal direction of radiation.

Although the devices of Figs. 3, 4, and 5 reduce the lobing of the radiation pattern of the arrangements of Figs. 1 and 2, they tend to be undesirably frequency sensitive with respect to if side lobing, unless extended at a wide and awkward angle.

The compact structure shown in Fig. 6 substantially reduces or entirely eliminates the side lobes .33 of Fig. 8 and minimizes frequency sensitivity. It is probable that in the structures of Figs. 3, 4, and 5 the frequency sensitivity is due to the connection between the adjunctive or auxiliary plates, such as the plates 36, 38 of Fig. 3 or the plates 49, 42 of Fig. 4 or the plates 48, 58 of Fig. 5 with the sides 28, 33 of the horn aperture 26. It is probable that this connection traps some energy, and is highly frequency sensitive because it may tend to resonate at some operating frequency.

This resonance effect may be avoided by using a structure like that of .Fig. 6 which is a cross- .sectional view of one desirable embodiment of 4 my invention. A pair of metallic reflectors 52, 54, preferably planar, are disposed closely adjacent (within a wavelength or less at the operating frequency) to the walls 28, 30 of the mouth 26, and on opposite sides respectively of these walls with respect to the central plane 32, and preferably parallel to the central plane. The reflectors 52, 54 extend only partially beyond the side walls 28 and 3B in the principal direction of radiation and are spaced from the plates 28 and 39 by dielectric spacers 56, 58 respectively. These spacers are tapered at the forward edges 56a and 561), respectively, to prevent the reflection of energy from these edges. Such tapering is also important to the reduction of the side lobing .and is a feature of the invention. It seems that too great an energy reflection from the space between the reflectors and the horn mouth side-walls causes the side lobing to persist. Thus, the tapering of the spacers is desirable for optimum results. An'y means to prevent energy reflection may be used, such as the use of an energy absorptive coating. However, tapering is simple and effective. The plates 28 and 30 terminate between the termination of spacers 56 and 58 and reflectors 52 and 54 as considered in the principal direction of energy radiation. The resonance inherent in Fig. 3, 4, or 5 is avoided 'by this arrangement of Fig. 6 and the side lobe reduction is not nearly so dependent on frequency.

Referring now more particularly to Fig. 7 which isa cross-sectional view of a preferred form of my invention, two substantially planar dielectric reflecting plates 6!] and 62, substantially parallel to the central plane 32, are disposed closely adjacent to the side walls 28, 30. They extend only partially beyond the termination thereof in the principal direction of radiation. The plates 60 and 52 are separated respectively from each wall portion or plate 28, 3!] in their closely spaced position by dielectric spacers 64 and 6B, respectively tapered at their forward edges, 64a and 66a, to avoid reflection. Spacers 64, vlifi'terminate behind the reflectors 6B, 62 and are maintained in place by clamps 69 and H placed at intervals along the Walls 28, 30 in the direction normal to the view of Fig. 7. The plates 28, 30 are attached respectively to the plates l4, it by a flanged arrangement as illustrated in the drawing. Struts 68 provide rigidity and physical strength to the construction. Eachof the reflectingplates 60, 62 have disposed, on the sides thereof opposite the central plane 32, energy absorptive material 10 thereby making one side of the reflectors E0, '62 substantially non-reflecting. The reflectors 60, 62 may have flanges l2, 14, respectively, to which may be attached a dielectric radome cover 16 whereby the aperture or mouth 26 may be made substantially weatherproof. When only one side of the dielectric reflectors 60, 62 is reflective .and when such reflection is not complete, an improved result is obtained in reducing .the sidelobe frequency sensitivity, as contrasted-withithe case when a metallic reflector such as the ,reflector of Fig. 6 is used giving complete reflection from the side exposed to the energy. With structures such as shown in Fig. 6 .or Fig. 7 a radiation pattern, illustrated in a somewhat idealized form by the heavy line 18 of Fig. 8, substantially complete elimination of the side lobes 33 is achieved.

The exact positioningand length of the reflecting plates may be determined experimentally. In general, it seems that-proper phasing of the reflected energy at a place close to the horn mouth aids in formation of the main lobe without side lobes. The invention is to be distinguished from arrangements where the side lobe energy is merely blocked or reflected at a place many or more wavelengths distant from the horn mouth, the latter arrangements requiring a cumbersome structure and operating on the theory of discarding all of the side lobe energy. Structure in accordance with the invention is more compact and utilizes the available energy more efflciently without the defects of the prior art arrangements.

It will be understood that the invention is equally applicable to aperture radiators in which, although a pillbox is not used, a similar mouth portion and sides similar to that of the pillbox in the mouth region are employed.

Thus the invention described comprises a compact, relatively non-frequency sensitive, eflicient horn radiator or receptor having a radiation or response pattern in which the side lobes are reduced to negligible values. The devices having such improved radiation patterns are as useful for the reception of energy as for the transmission thereof. Hence use of the terms radiator or receptor indicate that the embodiments of the invention may be used for either radiating or receiving electromagnetic energy, although the characteristics thereof as a radiator may completely describe the arrangement physically, and it has been so described herein.

What I claim is:

1. With an electromagnetic energy radiator or receptor having a wall portion defining a horn aperture and having a radiation pattern comprising a main lobe in a principal direction of radiation and side lobes, the combination comprising a reflector positioned outside said aperture and closely spaced adjacent said wall portion but dielectrically separated and insulated therefrom and extending partially and not entirely beyond the horn termination in the principal direction of energy radiation in said main lobe, whereby the side lobes are reduced or eliminated.

2. The combination claimed in claim 1, said reflector being dielectric and having energy absorptive'material on the side thereof remote from said aperture.

3. The combination claimed in claim 1, further comprising a non-reflective dielectric body between the reflector and the wall portion.

4. The combination claimed in claim 1, said reflector being metallic.

5. The combination claimed in claim 1, said reflector being metallic and having a substantially non-reflective dielectric spacer between said reflector and said wall.

6. With a radiator or receptor having a radiation pattern with a main lobe in the principal direction of radiation and with side lobes, and having an apertured horn having two flared side walls disposed on opposite sides of a central plane; the combination comprising a pair of substantially flat dielectric bodies substantially parallel to each other and to said central plane and spaced close to each of said side walls respectively on the side thereof away from said central plane and extending in the direction of main lobe radiation partially beyond the termination of said flared walls, whereby the side lobe or lobes are reduced or eliminated.

7. The combination claimed in claim 6, further comprising a pair of dielectric spacers respectively between each said body and the side wall near which it is closely spaced.

8. The combination claimed in claim 7, each said spacer being non-reflective of electromagnetic energy in the principal direction of radiation.

9. The combination claimed in claim 6, further comprising a pair of dielectric spacers respectively between each said body and the side wall near which it is closely spaced, each said spacer being tapered in the principal direction of radiation to be non-reflective of electromagnetic energy in said direction.

10. The combination claimed in claim 6, said aperture having said side walls flared in the plane of the electric vector of the energy to be radiated or received.

11. The combination comprising a radiator or receptor having a pair of substantially parallel walls normal to the electric vector of the energy to be radiated and spaced apart less than about a half free-space wavelength at the operating frequency, said walls each having a portion flared in the plane of the electric vector of the energy to be radiated, said portions being disposed on opposite sides of a central plane and together defining a horn aperture, said radiator having a radiation pattern with a main lobe in the principal direction of radiation and side lobes on each side of the said central plane, and a pair of substantially planar dielectric reflectors parallel to said central plane and closely spaced from said wall portions respectively on opposite sides thereof and extending in the said principal direction beyond the terminations .of said wall portions.

12. The combination claimed in claim 11, further comprising a pair of dielectric spacers one between each said dielectric reflector and the wall portion to which it is closely spaced, said wall portions terminating between the termination of said spacers and the termination of said reflectors in said principal direction of energy radiation.

13. The combination claimed in claim 12, said spacers being tapered in the principal direction of radiation to be non-energy reflecting in said direction.

14. The combination claimed in claim 11, each of said reflectors having energy absorptive material on the side thereof remote from. said central plane whereby only the side thereof adjacent said central plane is reflective of energy.

15. The combination claimed in claim 11, further comprising a dielectric cover over said aper ture suported by said reflectors.

WILLIAM C. WILKINSON, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,283,935 King May 26, 1942 2,293,839 Linder Aug. 25, 1942 2,349,440 Lavoie May 23, 1944 2,414,376 I-Ieim Jan. 14, 1947 2,429,640 Mieher Oct. 28, 1947 2,459,768 Cork Jan. 18, 1949 2,460,869 Braden Feb. 8, 1949 2,472,201 Eyges June 7, 1949 2,483,575 Cutler Oct. 4, 1949 2,495,219 Beck Jan. 24, 1950 FOREIGN PATENTS Number Country Date 890,388 France Feb. 7, 1944 

